Methods and apparatus for identifying asset location in communication networks

ABSTRACT

The location of unmodified wireless assets in a wireless communication network may be identified using time differences of arrivals of a communication sequence at different network receivers. Time-stamping devices may include correlator circuits in parallel with signal decoders to time-stamp communication sequences. Cellular wireless networks may be frequency-multiplexed to increase spatial time-stamping density. Tags may be attached to passive assets to provide location identification information to network devices. Locations of assets broadcasting standard 802.11 radio frequency structures may be identified. Noise inherent in correlating a communication sequence may be reduced by using a selected correlation function.

CROSS REFERENCE TO RELATED APPLICATION

This claims the benefit of U.S. Provisional Patent Applications Nos.60/270,254, filed Feb. 20, 2001, and 60/248,357, filed Nov. 14, 2000,both now abandoned which are hereby incorporated by reference herein intheir entireties.

BACKGROUND OF THE INVENTION

The present disclosure relates to apparatus and methods for tracking thelocation of assets in wireless or partially wireless communicationnetworks. In particular, the disclosure relates to identifying assetlocation using the time-of-arrival (hereinafter, TOA) at a receiver of acommunication sequence or sequences broadcast by a movable transmitter.TOA estimation may be referred to as “time-stamping” of a communicationsequence.

Precise estimation of communication sequence arrival times may bedesirable when mobile asset location is determined using ranging ortriangulation techniques in connection with receivers present in acommunication network. As the number of assets, the amount ofcommunication traffic, or the rate of time-stamping events in a networkincreases, time-stamping performance and overall network performance maydecrease. Usually, it is not desirable to identify the location of everywireless asset present in a network in connection with every broadcastpacket. Networks often operate inefficiently, however, because withouttime-stamping every received packet, it may be impossible to time-stampthe desired packets.

In some wireless networks, access point architecture may limit networkperformance. Often, access points do not permit time-stamping processesto be performed quickly enough to generate accurate estimates ofwireless asset location.

Wireless communication networks often have a cellular architecture inwhich adjacent cells operate on different channels. To minimizeinterference between communication signals, the cells are arranged tomaximize the physical distance between the channels. Although thisarrangement maximizes the spatial bandwidth available to network users,it often degrades location estimation accuracy by decreasing the densityof receivers available for time-stamping on a given channel.

Wireless communication networks are increasingly designed to use 802.11radio frequency signal structures. As this standard proliferates it willbecome increasingly valuable to identify the location of wireless assetsbroadcasting standard 802.11 packets.

Often, it is desirable to identify the location of “passive” assetspresent in the vicinity of a wireless communication network. Forexample, it may be desirable to track the location of pallets in a cargoyard. One solution is to attach active tags to the passive assets.Often, tags transmit specialized signals on a fixed schedule and on asingle frequency. When a network does not receive a scheduledtransmission, location information may be lost. Fixed schedule tags cannot “choose” when to transmit and so can not optimally utilize periodsof open “air time.” Location estimation may therefore be particularlydifficult or inefficient in high traffic communication networks. Singlefrequency tags may not be optimally trackable in multiple frequencycellular networks.

Time-stamping schemes often use correlation-based signal processingtechniques (similar to signal decoding techniques). Noise inherent inknown correlation techniques (e.g., cross-correlation artifacts) candegrade time-stamping accuracy.

Multiple signal arrivals (hereinafter, “multipath”) from a singlecommunication sequence transmission may degrade decoded signal qualityand make sequence detection difficult. Multipath may occur whenstructures near a transmitter produce transmission echoes that arrive atthe receiver after the “line-of-sight” signal. The “line-of-sight”(hereinafter, “LOS”) signal is the portion of a transmitted signal thattraverses the shortest path between transmitter and receiver. The LOSsignal may pass through structures. The LOS path may be opaque in thevisible spectrum. Multipath may contaminate decoded data signals withfalse data sequences and make detection of LOS data sequences difficult.Multipath may give rise to false sequences that have stronger signalsthan LOS signals because LOS signals may be attenuated by structuresthrough which they pass.

It would be desirable, therefore, to provide efficient apparatus andmethods for identifying wireless asset location.

It would also be desirable to provide apparatus and methods foraccurately identifying wireless asset location.

It would be further desirable to provide apparatus and methods foridentifying a location of a wireless asset broadcasting 802.11 signalstructures.

It would be still further desirable to provide apparatus and methods forefficiently tagging passive assets for location identification.

SUMMARY OF THE INVENTION

It is an object of this invention to provide improved apparatus andmethods for identifying wireless asset location in a wirelesscommunication network.

In accordance with the principles of the invention, apparatus andmethods for providing a time-of-arrival estimate of a data signal at areceiver may be provided. In some embodiments, the data signal may bereceived, demodulated, and decoded into a decoded signal. The decodedsignal may be analyzed for sequences favorable for estimating TOA. Iffavorable sequences are detected, a correlation function may be selectedfor correlating with the demodulated signal. The correlation functionmay be selected using rules that may be derived based on correlationproperties of data sequences. TOA may be estimated using the correlationfunction, values of the correlation function, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a schematic diagram showing an illustrative apparatus that maybe used in conjunction with a wireless asset location identificationsystem in accordance with the principles of this invention;

FIG. 2 shows another illustrative apparatus that may be used inconjunction with a wireless asset location identification system inaccordance with the principles of this invention;

FIG. 3 is a schematic diagram showing an illustrative communicationnetwork using apparatus such as those shown in FIGS. 1 and 2;

FIG. 4 is a flow chart showing illustrative steps that may be performedduring wireless asset location identification in accordance with theprinciples of the invention;

FIG. 5 is a schematic diagram of an illustrative communication networkarchitecture in accordance with the principles of the invention;

FIG. 6 is a another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 7 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 8 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 9 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 10 is a schematic diagram showing another illustrative apparatusthat may be used in conjunction with a wireless asset locationidentification system in accordance with the principles of thisinvention;

FIG. 11 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 12 shows illustrative received data and, in accordance with theprinciples of this invention, a corresponding illustrative correlationsignal;

FIG. 13 shows other illustrative received data and a correspondingillustrative correlation signal;

FIG. 14 shows two illustrative correlation signals in accordance withthe principles of this invention;

FIG. 15 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 15A shows an illustrative correlation signal that may be processedin accordance with the principles of the invention;

FIG. 16 is a flow chart showing illustrative steps that may be involvedin performing a step shown in FIG. 15;

FIG. 17 shows illustrative received data sequences and, in accordancewith the principles of the invention, illustrative correlationstrategies;

FIG. 18 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 19 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 20 shows illustrative apparatus that may be used in conjunctionwith a wireless asset location identification system in accordance withthe principles of this invention;

FIG. 21 is a schematic diagram of a portion of a wireless asset locationidentification system in accordance with the principles of thisinvention;

FIG. 22 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention;

FIG. 23 is yet another flow chart showing illustrative steps that may beperformed during wireless asset location identification in accordancewith the principles of the invention; and

FIG. 24 shows two illustrative correlation signals that may be used inwireless asset location identification in accordance with the principlesof the invention.

DETAILED DESCRIPTION OF THE INVENTION

Systems and methods for identifying a location of a wireless asset in acommunication network may be provided. Some embodiments of the inventionmay include a synchronization signal generator and network resourcesconfigured to time-stamp communication sequences broadcast by thewireless asset. Each network resource may be in electrical communicationwith the synchronization signal generator. Each resource may beconfigured to time-stamp the communication sequence.

The synchronization signal may be used to synchronize clocks present inthe network resources. Time-of-arrival estimates (hereinafter, TOAestimates) from the network resources may be used to identify a locationof the wireless asset. In some embodiments, the differences between TOAestimates for a communication sequence arriving at different networkresources may be used for location identification. Differences in TOA'sfor a communication sequence arriving at different network resources maybe referred to herein as “TDOA's”. A TDOA may be used to determine asolution set of possible wireless asset locations. A second TDOA may beused to identify an estimated asset location within the solution set. Insome embodiments, hyperbolic trilateration may be used to convert TDOA'sinto a wireless asset location estimate.

In some embodiments, an IEEE 802.11 communication sequence may betime-stamped. The communication sequence may include 802.11 radiofrequency signal structures. The communication sequence may include an802.11 packet.

Some embodiments of the invention may provide a TOA estimation devicefor providing a TOA estimate of a communication sequence at a locationin a wireless communication network. A TOA estimation device may includeone or more receivers for receiving one or more communication sequences.A TOA estimate may be an estimate of the time of arrival of acommunication sequence or a portion of a communication sequence. Acommunication sequence may include a series of information symbols. Aninformation symbol may be a PN code, a CCK symbol, a PBCC symbol, or anyother suitable information symbol. In a communication sequence, patternsof information symbols that have high quality autocorrelation propertiesmay be favorable for time-stamping. Patterns of information symbols thathave noisy autocorrelation properties may be unfavorable fortime-stamping.

The leading edge of a signal associated with a communication sequence ora portion thereof may be defined as a TOA. A peak in a correlationsignal (as discussed below) derived from a communication sequence or aportion thereof may be used to define a TOA. Any other suitable featureof a communication sequence or a portion thereof may be used to define aTOA. If multiple receivers in a network use the same TOA definition fora communication sequence, differences between the TOA's may be preciselyestimated.

In some embodiments, a TOA estimation device may include a device forreceiving radio frequency signals and a first circuit in electroniccommunication with the radio frequency device. The first circuit may beconfigured to detect a peak of a correlation function of a receivedcommunication sequence.

The first circuit may function as a correlator for detecting portions ofthe communication sequence that are favorable for TOA estimation. Thecorrelator may be a sliding correlator that multiplies the communicationsequence by a selected reference function. A product of thecommunication sequence and a selected reference function may be used todefine a correlation signal.

The first circuit may include a one-, two-, three-, four-, orfive-symbol correlator wherein each symbol in each correlatorcorresponds to an information symbol that may be present in a receivedcommunication sequence. A two-symbol correlator may be used to detect apattern of two consecutive corresponding symbols in the communicationsequence. A three-symbol correlator may be used to detect a pattern ofthree consecutive corresponding symbols in the communication sequence.Correlators of greater “length” may be used to detect longer symbolpatterns that may be present in the communication sequence.

The first circuit may include an N-symbol correlator, in which N is anypositive integer. The first circuit may include a combination ofcorrelators of different lengths. In some embodiments, the system mayinclude correlators arranged in series. In some embodiments, the systemmay include correlators arranged in parallel.

In some embodiments, a TOA estimation device may include a secondcircuit configured to decode the communication sequence received by areceiver. For example, in wireless 802.11b compatible networks, aone-bit (Barker sequence) correlator is often used as a decoder. (Asused herein, the term 802.11 may include any of the 802.11 family ofwireless communication network specifications, including 802.11a and802.11b.

In some embodiments, a TOA estimation device may include a third circuitconfigured to filter multipath signal components out of correlatoroutput signals (which may be referred to as correlation signals).Multipath signals are reflections of a communication signal fromstructures. Multipath signals are often received after the correspondingdirect or “line of sight” signal is received. Numerous multipath signalsmay be generated by reflections of a given line of sight signal. Themultipath signals may be stronger than the line of sight signals becauseline of sight signals are attenuated during propagation throughstructures. It may, therefore, be necessary to identify the line ofsight contribution to correlation signals.

In some embodiments, a TOA estimation device may include a fourthcircuit configured to output a signal indicative of a time-of-arrivalestimate. In some embodiments, a TOA estimation device may include afifth circuit configured to parse a mobile asset device identificationcode in said communication sequence. A mobile asset identification codemay include a Media Access Control (hereinafter, MAC) address. A mobileasset identification code may include an Internet Protocol (hereinafter,“IP”) address. The fifth circuit may perform conversions between MACaddresses and IP addresses. Conversions may be performed, for example,using an Address Resolution Protocol or a Reverse Address ResolutionProtocol. A mobile asset identification code may be selected by a systemadministrator. A mobile asset identification code may be an 802.11identification code. A mobile asset identification code may be a uniquecode. A mobile asset identification code may be a non-unique code.

In some embodiments, a TOA estimation device may include circuitry fordemodulating the communication sequence. In some embodiments, a TOAestimation device may include an antenna for receiving the communicationsequence. In some embodiments, a TOA estimation device may include acentral processing unit for executing tasks required for time-of-arrivalestimation. The central processing unit may be a personal computer. Insome embodiments, a TOA estimation device may include a radio module(such as those available under the names CompactFlash™ of CompactFlashAssociation, P.O. Box 51537, Palo Alto, Calif.; PC Card™ of PCMCIA, 2635N. First St. Suite 209, San Jose, Calif. 95134; and Mini PCI, a formfactor controlled by PCI S.I.G.).

In some embodiments, the invention may include a network of devices thatinclude receivers configured to selectively identify a mobile assetlocation by “selective listening.” The receivers may receive acommunication sequence from the mobile asset and optionally estimate thelocation of the asset if the asset has an identifier corresponding to aselected identifier. The selected identifier may be a unique identifier.The selected identifier may be a non-unique identifier. A selectedidentifier may be selected by a user of the system. A selectedidentifier may be stored in memory of a network resource. A networktransmitter may ping a selected wireless asset to cause the asset tobroadcast a communication sequence. The network may then time stamp thesequence.

In some embodiments, the invention may include a network of TOAestimation devices. Each device may include a receiver configured tooperate at a particular frequency. The network may include multiple TOAestimation devices configured to provide time-stamping of signalspropagating at different frequencies. A group of TOA estimation devicesmay be assigned to the same frequency. The assigned frequency of onegroup may be different from the assigned frequency of the other groups.The devices may be distributed in a cellular configuration. Methods foridentifying the location of a wireless asset that involve broadcastingor receiving on different frequencies may be referred to herein as“frequency multiplexing” methods.

The network may be configured to receive a communication sequence from awireless asset on a first frequency using one or more TOA estimationdevices from a first group. The network may then receive a communicationsequence from the wireless asset on a second frequency using one or moreTOA estimation devices from a second group. One or more additional TOAestimation device groups operating at one or more correspondingadditional frequencies may be used to receive one or more correspondingadditional communication sequences from the wireless asset. In someembodiments, the wireless asset transmissions may progress through allor some of the available network frequencies in a sufficiently shorttime period to ensure that the wireless asset location remainssubstantially unchanged during the period. Each TOA estimation devicemay time-stamp the communication sequence received on the respectivefrequency of the device.

One or more estimates of wireless asset location may be calculated fromTOA estimates produced by the TOA estimation devices. A TDOA may bederived from a pair of TOA's selected from the same or different groups.A TDOA may be derived from TOA estimation devices that are favorablylocated for identifying the mobile asset location. Favorably locateddevices may include those devices near the wireless asset. Favorablylocated devices may include devices that are distributed spatially in apattern favorable to location identification calculations (for example,in a triangular pattern surrounding the mobile unit).

In some embodiments, a preliminary location identification calculationmay be made to provide a coarse estimate of wireless asset location. Insome of these embodiments, a high-precision estimate may be made usingtime-of-arrival estimates from TOA estimation devices selected based onthe preliminary estimate of mobile asset location.

In some embodiments, a TOA estimation device may be configured toreceive communication signals on more than one frequency. For example,the device may be configured to receive communication sequences on allfrequencies available in the network. The device may include at leastone receiver configured to receive a communication sequence on eachfrequency available in the network. In some embodiments, all TOAestimation devices in a network may be configured to receivecommunication sequences on all available frequencies. In theseembodiments, a wireless asset operating on any one of the availablefrequencies may broadcast a communication sequence that may betime-stamped by all of the TOA estimation devices in the network.

In some embodiments that include TOA estimation devices configured toreceive communication sequences on multiple frequencies, the devices mayinclude a primary receiver and at least one auxiliary receiver. Theprimary receiver may be configured to receive a communication sequenceon an assigned frequency. The auxiliary receiver may be configured toalternate through a range of frequencies (e.g., all availablefrequencies in the network). The network may include a centralcontroller to force auxiliary receivers to operate at a selectedfrequency at a particular time. For example, the controller may causeall auxiliary receivers to simultaneously progress through a sequence ofdifferent frequencies. The different frequencies may include allavailable frequencies. During an interval in which all auxiliaryreceivers are switched to a particular frequency, potentially all TOAdevices in the network may be used to time-stamp a communicationsequence broadcast by a wireless asset on that frequency.

Some embodiments of the invention may include wireless assets configuredto broadcast a series of communication sequences for TOA estimation. Insome of the embodiments, the wireless asset may broadcast a series ofcommunication sequences using a series of different frequencies used bythe network receivers. In some of these embodiments, wireless assets maybe configured to repeatedly transmit a communication sequence for TOAestimation at a given frequency. Communication sequences that aretransmitted by a wireless asset for the purpose of identifying thelocation of the asset may be referred to herein as locationidentification information.

Some embodiments of the invention may include a tag that may be attachedto a mobile article that may be present in or near a communicationnetwork. Tags are described in U.S. Patent Application No. 60/248,357,filed Nov. 14, 2000, which is hereby incorporated by reference herein inits entirety. The tag may be configured to provide locationidentification information to the network. The location identificationinformation may include data in any format suitable for the network. Forexample, the tag may transmit 802.11 compatible data. In someembodiments, a tag may be configured to wait a predetermined period oftime, detect the presence of radio frequency energy (e.g., using anenergy detector) on a network channel, and, if the radio frequencyenergy is substantially less than a predetermined threshold, transmitthe location identification information to the network. The tag may waitin a “sleep” mode. The sleep mode may require reduced power. The sleepmode may be interrupted by a timer within the tag. The sleep mode may beinterrupted by a “wake-up” call from a network terminal.

The tag may be configured to switch to a different network frequency ifradio frequency energy on the first tested frequency is not less thanthe threshold. In some embodiments, the tag may continue to switch todifferent network frequencies until a clear channel is detected. When aclear channel is detected, the tag may transmit location identificationinformation to network receivers. In some embodiments, the tag may beconfigured to wait until a given channel is clear before broadcastinglocation identification information.

In some embodiments, the tag may be configured to transmit locationidentification information on multiple frequencies for frequencymultiplexing purposes. For example, a tag may include multipletransmitters to broadcast location identification information on morethan one channel in a cellular network. In some embodiments, a tag maybroadcast location identification information on multiple channelssequentially, for example, using a single tunable transmitter.

In some embodiments, the tag may transmit asset identificationinformation. Asset identification information may include informationidentifying the tag itself. Asset identification information may includeinformation identifying the mobile article to which the tag is attached.Asset identification information may include a MAC address, a portion ofa MAC address, an IP address, a portion of an IP address, or anysuitable unique or non-unique information for identifying the tag or themobile article to which it is attached.

In some embodiments, the tag may be configured to receive data from anetwork transmitter. In some of these embodiments, the tag may beconfigured to receive a wake-up signal from a network terminal.

The invention may include methods and/or apparatus for estimating a TOAof a communication sequence at a receiver. TOA estimation techniques arediscussed in U.S. Provisional Application No. 60/270,254, filed Feb. 20,2001, which is hereby incorporated by reference herein in its entirety.The communication sequence may be present in a data signal or a portionof a data signal. In some embodiments, the invention may include methodsfor receiving a data signal, demodulating the data signal (e.g., toyield a communication sequence), forming a decoded signal from the datasignal (e.g., to yield a bit sequence), and estimating a TOA of the datasignal using a selected or preselected correlation function.

In embodiments using a correlation function to estimate a TOA, thecorrelation function may comprise a representation of the communicationsequence and a selected reference signal. The correlation function maybe evaluated over a selectable portion of the data signal. The datasignal, which may be buffered as necessary, and the reference signal maybe combined in a way that allows the correlation function to have amaximum value when the reference signal most strongly correlates withthe data signal or a portion of the data signal. The time (relative tothe beginning of the data signal or any other temporal reference) atwhich such a maximum occurs may be defined as a TOA of the data signal.

The data signal may be a signal that is encoded to be recognized by aTOA estimation device. For example, a preselected communication sequencefavorable for time-stamping may be inserted into a data signal toimprove time-stamping accuracy. A TOA estimation device that recognizesthe encoded data signal may use a preselected reference signal togenerate a high quality correlation signal when correlated with thepreselected communication sequence. In embodiments of the inventionconfigured to receive known communication sequences, sequences of PNcodes, CCK symbols, PBCC symbols, or OFDM signals may be time-stamped.In these embodiments, the preselected reference signal may includesymbols selected from PN codes, CCK symbols, PBCC symbols, or OFDMsignals. A data signal may be encoded for time-stamping, for example, bysetting a symbol in a data signal header (e.g., an 802.11 packet header)or data packet to a predetermined value. In some embodiments, the datasignal may be encoded to instruct a TOA estimation device to time-stampthe sequence.

The data signal may be a signal that is not encoded to be recognized bya TOA estimation device. When a non-encoded signal is received, the datasignal may be monitored to detect communication sequences that may befavorable for correlation using one or more stored reference signals.When a potentially favorable communication sequence is detected, areference signal known to strongly correlate with the detected sequencemay be combined with the communication sequence to produce a correlationsignal. Embodiments of the invention that monitor data signals forfavorable communication sequences may detect sequences of Barker codes,PN codes, spectrum spreading codes (e.g., DSSS chipping codes), or anyother suitable codes.

In some embodiments of the invention, a TOA estimate may be determinedby maximizing or minimizing the value of the correlation function withrespect to a TOA parameter or estimator. For example, a TOA estimate maybe defined as the maximum likelihood estimate (or “peak”) of anindependent variable of the correlation function.

In some embodiments of the invention, a TOA estimate may be used tocalculate a time difference between arrival of a communication sequenceand a portion of a synchronization signal. In some embodiments, a TOAestimate may be used to calculate a time difference between acommunication sequence and a reference portion of a clock signal. Theclock signal may be generated by a clock internal to the TOA estimationdevice. The clock signal may be generated by a clock external to the TOAestimation device. In some embodiments of the invention, a TOA estimatemay be used to calculate a time difference between a communicationsequence received at a first receiver and the same communicationsequence received at a second receiver. In some embodiments, a TOAestimate may be used to calculate a time difference between a firstcommunication sequence and a second communication sequence. The firstand second communication sequences may be received at the same ordifferent receivers.

Some embodiments of the invention may provide for the filtering orremoval of multipath signal components from the correlation signal.Multipath may be removed by detecting the leading edge of a group ofpossibly overlapping peaks in a correlation signal. Multipath may beseparated from line of sight correlation signal components using channelestimation. In embodiments using channel estimation, the communicationchannel may be modeled as a series of discrete impulse functions inwhich the first impulse is assumed to be the line-of-sight impulse. Amultipath-free ideal correlation signal may be used in conjunction withthe actual correlation signal to construct an estimate of the channel.Using the estimated channel, line of sight and multipath impulses may beseparated.

In some embodiments of the invention, systems or system components forestimating a TOA of a communication sequence may be provided. Some ofthese embodiments may comprise apparatus for receiving, demodulating,decoding, buffering, processing, or filtering data signals. Someembodiments may comprise apparatus for outputting a TOA estimate toother network resources. Some embodiments may include multiplecorrelators. In some of these embodiments, correlators may be arrangedin parallel with each other.

Some embodiments of the invention may include circuits for estimating aTOA of a communication sequence at a receiver in a communicationnetwork. The circuitry may include a carrier tracking circuit. Thecircuitry may include a timing loop. The carrier tracking circuit may beused to adjust the oscillating frequency of a receiver used to receivethe radio signals.

The circuitry may include a chipping code correlator. The chipping codecorrelator may decode a spread spectrum communication sequence (e.g., adirect spread spectrum communication sequence) into a series of decodedbinary symbols. The chipping code correlator may use a correlationfunction to decode the communication sequence. The correlation functionmay include a sequence of symbols that match the chipping code of thecommunication sequence. The chipping code correlator may operate onsignals output from the carrier tracking circuit. Output from thechipping code correlator may be fed back to the carrier tracking circuitfor tracking circuit control.

The circuitry may include a time-stamping circuit. The time-stampingcircuit may use a correlation function or functions in conjunction withthe communication sequence to generate correlation signals. Thetime-stamping circuit may use a correlation signal for estimating thetime-of-arrival of the communication sequence. The time-stamping circuitmay include circuitry for separating multipath correlation signalcomponents from line of sight correlation signal components. Thetime-stamping circuit may operate on output from the carrier trackingcircuit.

The circuitry may include a receiver interface. The receiver interfacemay be a MAC interface. In some embodiments, a carrier tracking circuit,a chipping code correlator, and a receiver interface may be connected inseries. In some embodiments, the time-stamping circuit may receiveoutput from both the carrier tracking circuit and the correlator. Outputfrom the tracking circuit may be provided to the time-stamping circuitvia a correlator bypass. In some of these embodiments, output from thetracking circuit may be used in conjunction with output from thecorrelator to detect sequences in the communication sequence for use inTOA estimation. The output of the time-stamping circuit may be connectedto the receiver interface for communication with network resources.

Some embodiments may include a decoder circuit connected to the outputof the correlator circuit. The decoder circuit may decode the correlatorcircuit output to generate a series of binary information symbols, forexample, for low data communication rates. The decoder circuit may beconnected to the output of the carrier tracking signal. The decoder maydecode the tracking circuit output to generate a series of binaryinformation symbols, for example, for high data communication rates.

Some embodiments may include a descrambler circuit. The descramblercircuit may receive decoded signals and output descrambled signals tothe receiver interface.

The invention may include apparatus and/or methods for identifying alocation of a wireless asset using network resources configured totime-stamp a communication sequence broadcast by the asset. In someembodiments, the time-stamping network resources may include TOAestimation devices.

In some embodiments, a TOA of a communication sequence broadcast by awireless asset may be estimated by a TOA estimation device. The TOA maybe estimated using a selected correlation function. The TOA of the samecommunication sequence may then be estimated at a second TOA estimationdevice.

The TDOA (time-difference-of-arrivals) of the two TOA's may be used todefine a set of possible locations from which the communication sequencemay have been transmitted. Techniques for identifying a location of awireless asset using time-differences-of-arrivals are described in U.S.Provisional Patent Application No. 60/248,357, filed Nov. 14, 2000,which is hereby incorporated by reference herein in its entirety. Theset may be a hyperbolic curve. In some embodiments, each TOA estimationdevice may have an internal clock, or counter, for quantifying a TOA.Clocks that may be included in TOA estimation devices in a network maybe synchronized or calibrated to permit the calculation of TDOA'sderived from TOA's acquired at different TOA estimation devices.

A TDOA generated by a first TOA estimation device pair may be referredto herein as the first TDOA. A set of possible locations correspondingto the first TDOA may be referred to as a first solution set. Additionallocation information may used in conjunction with the first solution setto identify the location of the asset. In some embodiments, theadditional location information may be acquired sufficiently rapidly toensure that only small changes in the location of the asset may occurduring acquisition of location identification information at TOAestimation devices.

Additional location information may include, for example, at least oneadditional TDOA solution set. The additional TDOA solution set maydefine a second hyperbolic curve that intersects with the firsthyperbolic curve. The additional TDOA solution set may be derived fromTOA estimation devices that are different from the TOA estimationdevices of the first pair. The additional TDOA solution set may bederived from one TOA from the first pair and one TOA from a TOAestimation device not included in the first pair. Many additional TDOAsolution sets may be used. The identity of the asset location may bedefined as the most likely or most precisely estimated intersection ofthe two solution sets. In some embodiments, asset location may beidentified using a least squares estimate of the intersection. In someembodiments, asset location may be identified using a maximum likelihoodestimator of the intersection.

Additional location information may include a distance (or range)estimate from a TOA estimation device external to the original TOAestimation device pair. Range may be determined by receiving acommunication sequence from the mobile asset when both send- andreceive-time are known. Range may be determined by transmitting a signalto the mobile asset from the TOA estimation device, receiving an “echo”signal from the mobile asset, and calculating the range using the roundtrip travel time. For example, in an 802.11 communication network, theecho signal may be an 802.11 acknowledgment frame (an “ACK”). In someembodiments, a delay between receipt of the transmitted signal andbroadcast of the echo signal may be precisely controlled to provide aprecise round-trip travel time estimate. Range may be determined byreceiving a communication signal from the wireless asset and estimatingrange using signal strength attenuation. Range may be determined usingany other suitable means.

Additional location information may include physical, geographic, orgeometric restrictions on the location of the mobile asset. For example,the first TDOA may pass through several sectors of the network. If it isknown that the mobile asset is not present in one or more of thesectors, those sectors may be ruled out as possible locations and theremaining sectors may be used to identify the asset location.

In some embodiments of the invention, TDOA may be defined as the averageof multiple differences between TOA's from a given pair of TOAestimation devices. For example, a first TOA estimation device mayreceive a communication sequence from a mobile asset. The first TOAestimation device may generate three different TOA's. A second TOAestimation device may receive the communication sequence from the mobileasset and generate three corresponding TOA's. Three TDOA's may begenerated by calculating differences from the three pairs ofcorresponding TOA's. The three TDOA's may then be averaged and definedas an effective TDOA. The successive TOA's at each TOA estimation devicemay be generated by repetitive application of the same correlationfunction to the communication sequence, application of differentsuccessive correlation functions, by the application of a singlecorrelation function that generates multiple TOA estimators (see below),or by any other suitable method.

In some embodiments, when multiple TDOA's are generated for a receivedcommunication sequence (for example, when multiple independent TOAestimation device pairs receive the communication sequence), it may benecessary to find the intersection of more than two solution sets. Whenmore than two solution sets are present, asset location may bedetermined by using least squares, maximum likelihood, or any othersuitable method for estimating the most likely identity of the location.

When more than two solution sets are present, it may be necessary todiscard one or more of them. For example, in some embodiments a noisysolution set may be discarded if noise associated with underlying TOAdata is above a predetermined threshold. Some embodiments may discard asolution set that is an “outlier” relative to other solution setsderived for the same communication sequence. In embodiments that usemaximum likelihood estimation to determine location identity, eachsolution set may be weighted according to the amount of noise present inthe solution set. In this manner, more noisy TDOA solution sets may bediscounted relative to less noisy TDOA solution sets.

Some embodiments of the invention may include a timing cable forsynchronizing TOA estimation device clocks in a network. In some of theembodiments, the cable may provide a high frequency sine wave. In someof these embodiments, the cable may provide a high frequency squarewave. The synchronization signal may be generated by a network resource.The synchronization signal may be multiplied by the TOA estimationdevice, for example, using a phase-locked loop. At the TOA estimationdevice, the signal may be amplified, filtered, wave-shaped, or processedin any other suitable manner. The signal may be processed to produce adigital signal configured to drive a digital counter. The digitalcounter may serve as a TOA clock for TOA estimation devices in acommunication network.

In some embodiments, TOA estimation device clocks in a network may besynchronized by periodically modulating the synchronization signal. Aperiodic modulation may be used as a global clock reset. A demodulatorin a TOA estimation device may be used to detect the periodicmodulation. The demodulator may reset the digital counter. In someembodiments, the periodic modulation may be accomplished by removing thehigh frequency components of the signal for a selected number of cyclesor pulses. A re-triggerable one-shot with a timeout greater than asingle pulse may be used to detect the missing pulses and generate theclock reset. A TOA estimation device may include a large counter. Forexample, the counter may have as many as 32 bits. In some embodiments,the counter may have greater than 32 bits.

Some embodiments of the invention may include a calibration process tocompensate for differing fixed delays associated with individual TOAestimation devices. These delays may include, but are not limited to,delays in receivers and cables. Delays may be quantified and used toadjust wireless asset location estimates calculated using TOA estimatesgenerated by the TOA estimation devices. In some embodiments, delays maybe stored in memory.

Illustrative examples of embodiments in accordance with the principlesof the present invention are shown in FIGS. 1–24.

FIG. 1 shows illustrative TOA estimation device 100 including receiver110 and processor 120. TOA estimation device 100 may include atransmitter for transmitting signals to a wireless asset such as asset130. In some embodiments, TOA estimation device 100 may be, or may bepart of, a wireless network access point such as an 802.11 compatibleaccess point or any other suitable access point. In some embodiments,device 100 may not include components normally associated with an accesspoint, such as a transmitter. Receiver 110 may receive communicationsignal 112 from a wireless asset such as 130. Wireless asset 130 may bea mobile personal computer, palmtop computer, handheld personal compute,automobile personal computer, personal digital assistant (PDA), cellularphone, cellular phone/PDA combination, wireless tag, wireless shoppingappliance, wireless inventory appliance, or any other device suitablefor transmitting a wireless communication signal. Wireless asset 130transmit communication signals via wires.

Receiver 110 may include any hardware, firmware, or software necessaryfor receiving, demodulating, and decoding communication signal 112 fromwireless asset 130. In some embodiments, signal processing tasks may bedistributed or shared between receiver 110 and processor 120. Forexample, receiver 110 may demodulate communication signal 112 andprocessor 120 may perform decoding and TOA analysis tasks. In someembodiments, processor 120 may receive a communication signal 112 fromreceiver 110, demodulate the signal, and carry out TOA analysis tasksusing suitable signal processing and/or analysis hardware or software.In some embodiments, the tasks of receiver 110 and processor 120 may beintegrated into a single component (e.g., in an access point).

Processor 120 may be associated with, for example, a personal computer,palmtop computer, handheld personal computer, automobile personalcomputer, personal computer, personal digital assistant (PDA), cellularphone, cellular phone/PDA combination, set-top box, portable computer,internet server, network server, thin server, or any other devicesuitable for processing communication signals or supporting signalanalysis tools.

User 140 may be in communication with system 100 via any suitable wiredor wireless means. In some embodiments of system 100, a user such as 140may interact with processor 120 via a keypad, keyboard, touchpad, or anyother suitable interface. User 140 may be a client or host processor.

FIG. 2 shows illustrative TOA estimation device 200, which may providefunctions similar to those described in connection with TOA estimationdevice 100. TOA estimation device 200 may include antenna 210. Antenna210 may receive wireless signals from a wireless asset. In someembodiments, antenna 210 may transmit wireless signals to a wirelessasset. In some embodiments, TOA estimation device 200 may communicateTOA estimation information to a user via wired media. In someembodiments, TOA estimation device 200 may communicate TOA estimationinformation to a user via wireless media. In some embodiments, TOAestimation device 200 may not include transmission capabilities.

In some embodiments, central processing unit 220, which may interactwith radio module 230 and/or high resolution timing module 240, maydemodulate signals received by antenna 210, decode the signals, performserial or parallel correlation tasks, perform hybrid serial-parallelcorrelation tasks, and provide buffering, data manipulation, and dataformatting as necessary to generate or output TOA estimates for acommunication sequence that system 200 receives. Processing unit 220 mayuse any suitable signal processing and/or analysis hardware or software.

In some embodiments of the invention, radio module 230 may providein-phase and quadrature radio signal components, I and Q, respectivelyto timing module 240, which may be a high resolution timing module.Radio module 230 provide auxiliary signals to timing module 240. Forexample, radio module 230 may provide RSSI signal (e.g., for measuringsignal strength), MD-RDY signal (e.g., for framing data), RXCLK signal(e.g., for clocking data), and RXD signal (e.g., for communicatingreceived data).

In some embodiments, timing module 240 may include data acquisitionmodule 241, RSSI A/D module 242, timer module 243, asset ID parsermodule 244, configuration registers 245, correlation module 246, and anyother suitable modules. Data acquisition module 241 may acquirecommunication sequences from signals I and Q. Data acquisition module241 may convert an analog signal to a digital signal. Timer module 243may include an oscillator and a counter. Timer module 243 may provide atime reference for time stamping a communication signal received bysystem 200. Timer module 243 may be used to time-stamp signalstransmitted by radio module 230. Time-stamping of a transmitted signalmay be used in conjunction with a TOA of a received signal to estimateround trip travel time of a signal between TOA estimation device 200 anda wireless asset. Timer module 243 may be synchronized with other timermodules in TOA estimation devices that may be present in a network sothat a difference between two TOA's received at different TOA estimationdevices may be calculated. In some embodiments, the timer modules mayreceive a synchronization signal from a network resource. Asset IDparser module 244 may be present, for example, for parsing wirelessasset identification information that may be present in the receivedcommunication sequence (e.g., a MAC address). Parser module 244 mayinclude data filters to parse the identification information.Configuration registers 245 may be present. Configuration registers 245may be used, for example, for storing wireless asset identificationinformation for selective time-stamping (e.g., as discussed above).Correlator module 246 may be present for generating TOA estimates byapplying correlation functions to communication sequences. Correlatormodule 246 may include multipath processing components.

In some embodiments, timing module 240 may provide an asset ID, a TOAestimate, I/Q data, RSSI, or any other suitable information to centralprocessing unit 220. Unit 220 may process information received fromtiming unit 240. Unit 220 may provide information such as asset ID, aTOA estimate, I/Q data, RSSI, signal strength, round trip propagationtime, distance to a wireless asset, or any other suitable information toother network resources by wireless or wired means. Unit 220 may providean asset ID and a TOA to a centralized asset location identificationprocessor for the calculation of TDOA's.

FIG. 3 shows TOA estimation device 300, which may be similar to TOAestimation devices 100 and 200, integrated into communication network310. Communication network 310 may be supported by network server 320.Communication network 310 may be a local area network, wide areanetwork, a metropolitan area network, an intranet, an extranet, anyother type of communication network, or any wireless or partiallywireless form of any such network. Network server 320 may be incommunication with internet 330 or any other electronic communicationnetwork. Network 310 may include wired assets 340. Features such as TOAestimation device 300, server 320, Internet 330, wired assets 340, andwireless assets 350 may be referred to herein as network resources. Awired asset 340 may be a personal computer, palmtop computer, handheldpersonal computer, a personal data assistant, a set-top box, a portablecomputer, an Internet server, a LAN server, a thin server, or any othersuitable processing device. One or more of TOA estimation devices 300may estimate the TOA of signals received from wireless assets such as350. Network server 320, one of wired assets 340, one of TOA estimationdevices 300, or any other suitable network resource may generate asynchronization signal for synchronizing clocks that may be present inTOA estimation devices 300.

FIG. 4 shows a generalized flowchart of illustrative steps that may beinvolved in some embodiments of the present invention related toidentifying a location of a wireless asset in a communication network.The steps shown in FIG. 4 are only illustrative and may be performed inany suitable order. In practice, there may be additional steps or someof the steps may be deleted. Some of the steps shown in FIG. 4 mayinvolve receiving wireless signals. Some of the steps shown in FIG. 4may involve transmitting wireless signals. Some of the steps shown inFIG. 4 may involve processing signals. These and other steps may beperformed using any suitable apparatus, including some or all of theelements of system 100 shown in FIG. 1, system 200 shown in FIG. 2, andsystem 310 shown in FIG. 3.

In step 410, a TOA estimation device or a group of TOA estimationdevices may receive at least one communication sequence from a wirelessasset. In some embodiments, communication sequences may be received byTOA estimation devices operating at different frequencies. In step 420,one or more communication sequence may be time-stamped. In someembodiments, the time-stamping may be performed locally at the TOAestimation device. In step 422, a central processor may collect TOAestimates from network TOA estimation devices and calculate TDOA's forpairs of TOA's. In step 424, TDOA's may be used to generate wirelessasset TDOA solution sets for identifying a wireless asset location (forexample, using hyperbolic trilateration).

In step 426, a TOA may be used in conjunction with a time of broadcast(hereinafter, “TOB”) of a ranging signal to estimate the distancebetween a TOA estimation device and a wireless asset. For example, aTOB, a TOA, and any delays involved in signal processing may be used tocalculate a round trip signal propagation time. Using the propagationspeed of the signal, the distance may be estimated.

In some embodiments, signal strength of a received communicationsequence may be estimated in step 430. In some of these embodiments,signal strength may be estimated locally at a TOA estimation device. Insome embodiments, a signal strength at a receiver may be used inconjunction with a signal strength at a wireless asset transmitter tocalculate signal attenuation. In step 426, signal strength orattenuation may be used to estimate a distance between the wirelessasset and a TOA estimation device. In step 428, estimates of distancesfrom different receivers of known position may be used to identify awireless asset location, for example, using triangulation.

In step 440, communication sequence carrier signal phase may be used toestimate an angle of arrival (hereinafter, “AOA”) of a communicationsequence at a TOA estimation device. When more than one AOA is known fora communication sequence or a series of communication sequences from awireless asset, a wireless asset location may be identified using theintersection of two or more carrier signal propagation directionvectors.

In some embodiments, wireless asset location may be identified usinghybrid methods combining a TDOA solution set, a distance betweenwireless asset and TOA estimation device, a carrier signal propagationdirection, or any combination or sub-combination thereof.

FIG. 5 shows the architecture of illustrative cellular wirelesscommunication network 500 that may be used to identify a location of awireless asset using frequency multiplexing. Network 500 may includesome or all of the elements of network 310 shown in FIG. 3. Network 500has numerous cells operating at selected frequencies. The frequenciesshown in the cells of network 500 are primary operating frequencies(e.g., “assigned” frequencies). In some embodiments, TOA estimationdevices may operate on at least one frequency that is different from theprimary frequency. Each cell may have one or more receiver devices oraccess points. In some embodiments, each cell may have one or more TOAestimation device such as 100. Network 500 has three primary operatingfrequencies, f₁, f₂, and f₃, but the invention may include networks thathave a greater or lesser number of primary operating frequencies.Wireless asset 520, positioned within an f₃ cell, is within range ofnearby receiver devices operating at frequencies f₁, f₂, and f₃. If oneof the three operating frequencies is “busy” with other communicationtraffic, wireless asset 520 may attempt to communicate using at leastone of the other two frequencies.

In some embodiments, TOA estimation devices in each cell may beconfigured to transmit and receive primarily on the operating frequencyassigned to the cell. If wireless asset 520 broadcasts a communicationsequence on f₁, for example, TOA estimation devices located in cells D,E, and B may receive and time-stamp the communication sequence. In someembodiments of the invention, wireless asset 520 may broadcast thecommunication sequence on at least one additional frequency to permittime-stamping at other (e.g., more proximal) TOA estimation devices. Forexample, wireless asset 520 may broadcast the communication sequencefirst on frequency f₁, second on frequency f₂, and third on frequencyf₃, thereby allowing devices in nearby cells A, B, C to time-stamp thecommunication sequence. In some embodiments, wireless asset 520 maybroadcast the communication sequence on multiple frequenciessimultaneously using parallel transmitters. In some embodiments,wireless asset 520 may broadcast the communication sequence successivelyon multiple frequencies.

In some embodiments, the TOA estimation devices in a cell may beconfigured to switch from a primary frequency to a secondary frequency,a tertiary frequency, or a different frequency. TOA estimation devicesin a cell may switch from one frequency to another to receive andtime-stamp communication sequences from a wireless asset configured tobroadcast, for example, on a single frequency (which may be referred toherein as a “target” frequency). In some embodiments, some or all of theTOA estimation devices in a network may be configured to switchsubstantially simultaneously to a target frequency to receive acommunication sequence for time-stamping.

FIGS. 6–9 show generalized flowcharts of illustrative steps that may beinvolved in identifying a location of a wireless asset in acommunication network using frequency multiplexing. The steps shown inFIGS. 6–9 are only illustrative and may be performed in any suitableorder. In practice, there may be additional steps or some of the stepsmay be deleted. Some of the steps shown in FIG. 6 may involve generatingand/or transmitting wireless signals using a wireless asset and may beperformed by any suitable apparatus. For example, some or all of thesteps shown in FIG. 6 may be performed using a wireless asset such aswireless asset 130 shown in FIG. 1. Some of the steps shown in FIGS. 7–9involving receiving wireless signals or processing signals may beperformed using any suitable apparatus, including some or all of theelements of system 100 shown in FIG. 1, system 200 shown in FIG. 2,network 310 shown in FIG. 3, and network 500 shown in FIG. 5.

In step 610 shown in FIG. 6, the wireless asset may wait a selectedperiod of time before broadcasting a signal that may be used fortime-stamping by receiver devices in a communication network. Thenetwork may be similar to network 310 shown in FIG. 3 or network 500shown in FIG. 5. In embodiments involving a multifrequency cellularnetwork such as network 500, the wireless asset may broadcast acommunication sequence on frequency f₁ (in step 620) at the end of theselected time period. In step 630, the wireless asset transmitter mayswitch to frequency f₂ and broadcast a communication sequence. In step640, the wireless asset transmitter may switch to frequency f₃ andbroadcast a communication sequence.

In some embodiments, the wireless asset transmitter may switch toadditional frequencies, as denoted by f_(2+n) in step 640, correspondingto operating frequencies available in the communication network. In someembodiments, the communication sequences broadcast on the differentfrequencies may be substantially identical. In some embodiments, thecommunication sequences broadcast on the different frequencies may bedifferent.

FIG. 7 shows illustrative steps that may be used in a frequencymultiplexing scheme in which groups of TOA estimation devices, eachoperating at a frequency assigned to the group, may be used to identifya location of a wireless asset in a communication network. For the sakeof illustration, the network may be divided into three groups of TOAestimation devices (Group I, Group II, and Group III). In someembodiments, the network may be divided into a smaller number of groups.In some embodiments, the network may be divided into a larger number ofgroups. In step 700, the network may receive using Group I devices acommunication sequence broadcast on frequency f₁ by the wireless asset.In step 705, Group I devices may time stamp the communication sequence.In step 710, a network processor may calculate TDOA's from pairs ofTOA's estimated in Group I. In step 715, a solution set including one ormore possible wireless asset locations for each TDOA derived from GroupI TOA's may be generated.

In step 720, the network may receive using Group II devices acommunication sequence broadcast on frequency f₂ by the wireless asset.Processes in steps 725–735, involving Group II devices and acommunication sequence received on frequency f₂, may be analogous to theprocesses in steps 705–715, involving Group I devices and acommunication sequence received on frequency f₁. In step 740, thenetwork may receive using Group III devices a communication sequencebroadcast on frequency f₃ by the wireless asset. Processes in steps745–755, involving Group III devices and a communication sequencereceived on frequency f₃, may be analogous to the processes in steps705–715, involving Group I devices and a communication sequence receivedon frequency f₁. In step 760, a network processor may identify thelocation of the wireless asset using one or more of the solution setsgenerated in steps 715, 735, 755. If only one solution step is generatedin steps 715, 735, and 755, collectively, more information may berequired to identify the wireless asset location. Step 760 may befollowed by a return to step 700 to begin a new multi-frequencytime-stamping cycle.

FIG. 8 shows illustrative steps that may be used in a frequencymultiplexing scheme in which TOA estimation devices including one ormore auxiliary receivers may be used to identify a location of awireless asset in a communication network. An auxiliary receiver may bepresent in a TOA estimation device (e.g., in addition to a primaryreceiver operating at a primary frequency) to provide signal receptionon a frequency other than a primary operating frequency withoutinterrupting reception on the primary frequency. In step 820, auxiliaryreceivers operating at a uniform frequency may receive a communicationsequence from a wireless asset. In step 830, the TOA of thecommunication sequence at each receiver may be estimated (e.g., using aTOA estimation device associated with each receiver). In step 840,TDOA's may be calculated from pairs of TOA's estimated at the receivers.In step 850, a solution set including one or more possible wirelessasset locations for each TDOA derived in step 840 may be generated. Instep 860, the wireless asset location may be identified using one ormore of the solution sets generated in step 850. If only one solutionset is generated in step 850, additional information may be required toidentify the wireless asset location. In step 870, the auxiliaryreceivers may be switched to a new uniform frequency to repeat thewireless asset location cycle using a communication sequence which maybe received on the new frequency. In some embodiments of the invention,auxiliary receivers may not be present. In these embodiments, primaryreceivers may be configured to be switched to different frequencies toperform steps 820–870.

FIG. 9 shows illustrative steps that may be performed by a network forcarrying out “selective listening” for wireless assets. In someembodiments, a communication sequence received from a wireless assetthat is a member of a preselected set of wireless assets may betime-stamped. In step 910, one or more wireless asset identifierscorresponding to wireless assets for which location identification isdesired may be selected. In step 920, selected identifiers may bestored. In step 930, a communication sequence from a wireless asset maybe received by the network. In step 940, an asset identifier may beparsed from the communication sequence to determine the identity of thetransmitting wireless asset. In step 950, the received identifier may becompared to preselected identifiers. If the received identifier does notmatch a preselected identifier, no TOA or location identificationprocessing may occur. The process may revert to step 930 and the networkmay receive a new communication sequence for possible locationidentification. If the received identifier matches a preselectedidentifier, the communication sequence TOA may be estimated in step 960.Steps 930–960 may be performed at multiple receiver devices in thenetwork so that multiple TOA's respectively corresponding to themultiple receiver devices may be estimated in step 960. In step 970, theTOA estimates from step 960 may be used to identify the wireless assetlocation. In some embodiments, steps 960 and 970 may involve estimatingand/or processing non-TOA quantities that may be used for locationidentification. For example, steps 960 and 970 may include one or moreof steps 424, 426, 428, 430, 440, and 442 shown in FIG. 4.

FIG. 10 shows an illustrative architecture for wireless tag 1000 thatmay be attached to a mobile article for location identificationpurposes. Antenna 1010 may transmit signals to and receive signals fromapparatus (such as system 100 shown in FIG. 1 and system 200 shown inFIG. 2) of a communication network. Radio transmitter 1020 may bepresent to generate communication sequences for broadcast to thenetwork. Transmitter 1020 may be configured to modulate communicationsequences using carrier signals of different frequencies. In someembodiments, communication sequences that are favorable for wirelessasset location identification may be selected. Wireless assetidentification information may be included in some communicationsequences. Controller 1040 may be present in tag 1000 and may interactwith radio receiver 1030 and clear channel detector 1060 to detect thepresence of radio frequency traffic on a communication channel. In someembodiments, radio receiver 1030 may not be present. In theseembodiments, clear channel detector 1060 may be used to determine if achannel is clear. Clear channel detector 1060 may be an energy detector.Tag 1000 may “listen” to traffic on a channel using receiver 1030 anddetector 1060. If the channel is clear, tag 1000 may broadcast acommunication sequence on the channel. If traffic is present on thechannel, controller 1040 may switch receiver 1030 to successivedifferent listening frequencies until a clear channel is detected.Controller 1040 may then switch transmitter 1020 to the clear channel tobroadcast the communication sequence. Transmit/receive switch 1050,which may be controlled by controller 1040, may be present for switchingantenna 1010 into communication with either transmitter 1020 or receiver1030.

In some embodiments, receiver 1030 may be configured to receive a signalfrom the network that includes an instruction to broadcast acommunication sequence to enable the network to identify the location ofthe tag. Tag 1000 may be powered by power supply 1080. In someembodiments, component group 1070 may be configured to operate at lowpower to reduce the load on power supply 1080.

FIG. 11 is a generalized flowchart of illustrative steps that may beinvolved in providing location identification information concerning amobile article to a communication network. The steps shown in FIG. 11are only illustrative and may be performed in any suitable order. Inpractice, there may be additional steps or some of the steps may bedeleted. Some of the steps shown in FIG. 11 may involve generatingand/or transmitting wireless signals using a tag and may be performed byany suitable apparatus. For example, some or all of the steps shown inFIG. 11 may be performed using a wireless asset such as wireless asset130 shown in FIG. 1 (including tag 1000 shown in FIG. 10), or anysuitable apparatus. For the sake of simplicity, it will be assumed thatthe steps shown in FIG. 11 are performed by tag such as 1000.

In step 1110, a tag may be maintained in a sleep mode. In someembodiments of the invention, the sleep mode may conserve power, forexample, in power supply 1080 shown in FIG. 10. A transmitter, which maybe a network transmitter, may transmit a wake-up signal to the tag. Awake-up signal may be a strong RF signal transmitted near the tag. Instep 1120, the tag may receive the wake-up signal using an energydetector. If no wake-up signal is received by the end of a predeterminedtime period, the tag may be configured to wake-up automatically in step1130. Transmitter and receiver circuitry (such as that in componentgroup 1070 shown in FIG. 10) may be energized in step 1125. If a wake-upsignal is received, transmitter and receiver circuitry may be energizedin step 1125. After energizing transmitter and receiver circuitry, thetag may determine if a communication channel is clear for broadcasting acommunication sequence in step 1140. In some embodiments, step 1140 maybe performed using an energy detector. Some of the steps shown in FIG.11 may be performed by a tag (e.g., a tag similar to tag 1000 shown inFIG. 10) that is configured to broadcast 802.11 signals, but is notfully 802.11 compliant. For example, the tag may not include a receiver.If the channel is not clear for broadcast, the tag may activate a delayin step 1150. Step 1150 may include the use of a back-off algorithm toprovide a preselected delay before returning to step 1140 to check thechannel again. The back-off algorithm may be an 802.11 compliantback-off algorithm. In some embodiments, step 1150 may be followed by areturn to step 1110 to return the tag to sleep mode. In someembodiments, step 1140 may include detecting traffic on successivedifferent frequencies.

When a clear channel is detected in step 1140, the tag may broadcast acommunication sequence in step 1160. The communication sequence may bebroadcast on the clear channel detected in step 1140. The communicationsequence broadcast in step 1160 may include information that identifiesthe tag or the article to which it is attached. Any suitableidentification information may be included in the communicationsequence, for example, if the step is performed by a different type ofwireless asset. The communication sequence may include locationidentification information. The communication sequence may includesymbols compatible with IEEE 802.11 communication standard.

In step 1170, the tag may uplink data to the network. The tag may uplinkdata to the network using 802.11 communication protocols. Uplinked datamay include battery status information, tag temperature information, orany other suitable information. In some embodiments, the tag maybroadcast a communication sequence on a different channel in step 1180.If so, the process may revert to step 1140 and a new clear channel maybe sought. If not, the process may revert back to 1110 to return the tagto sleep mode.

FIGS. 12–20 illustrate some of the principles, methods, and apparatusthat may be involved in embodiments of the invention that may providecommunication sequence time-stamping.

A correlation function may be used to detect patterns in the symbolspresent in a communication sequence. A local or global extreme value ina correlation function may correspond to a symbol pattern having strongautocorrelation properties. A symbol pattern having strongautocorrelation properties may generate an easily observed andreproducible correlation function peak. The time value of such a peak beused as a TOA estimate, or a time stamp, for the received data signal.In some embodiments, a correlation function C(τ) may be defined by:C(τ)=∫_(−T) ^(T) D(t)R(t−τ)dt  (1)wherein t is a measure of time, D(t) represents a demodulated receivedsignal which may be time-dependent, and R(t) represents a referencesignal. R(t) may correspond to a pattern of symbols present in D(t). −Tand T, respectively, may be the beginning and end of a time intervalduring which C(τ) is evaluated (or scanned for an extreme value). TheTOA estimate for D(t) may be defined as the value of τ that causes C(τ)to have an extreme value. The extreme may be a maximum. The extreme maybe a minimum. The value of τ that corresponds to an extreme value inC(τ) may be referred to as {circumflex over (τ)}.

FIG. 12 shows an example of C(τ) for illustrative examples of D(t) andR(t). In this example, D(t) is a communication sequence that includes aconcatenation of three consecutive identical information symbols. Eachsymbol, indicated by a “+”, is illustrated as a PN code. For example,the PN code may be a Barker code. Although the symbols are illustratedas PN codes, principles discussed herein may be applied to PBCC, CCK,OFDM, or other suitable symbols. R(t) may be chosen to correspond to aninformation symbol or pattern of information symbols that may be presentin D(t). In the example shown in FIG. 12, R(t) is the same as the singlerepeating information symbol (“+”). For each occurrence of R(t) in D(t),C(τ) has a peak value. Any of estimators {circumflex over (τ)}₁,{circumflex over (τ)}₂, and {circumflex over (τ)}₃, corresponding tolocal C(τ) peaks C₁, C₂, and C₃, respectively, may be selected as a TOAestimate. In some embodiments, the TOA of D(t) may be defined to be theaverage of estimators such as {circumflex over (τ)}₁, {circumflex over(τ)}₂, and {circumflex over (τ)}₃. For example, the TOA of D(t) may bedefined as:τ

$\begin{matrix}{\left\langle \hat{\tau} \right\rangle \equiv {\frac{1}{N}{\sum\limits_{i}^{N}{\hat{\tau}}_{i}}}} & (2)\end{matrix}$wherein {circumflex over (τ)} is the average of N {circumflex over (τ)}estimates for a given D(t).

FIG. 13 shows an example of D(t) that is similar to that shown in FIG.12, but the “polarity” of the second information symbol, identified by a“−” in FIG. 12, is reversed. (Information symbols of opposite“polarity,” as used herein, produce correlation signals of opposite signfor a given correlation function.) The corresponding second peak in C(τ)has a negative value. Because the value of C(τ) at a given point in timedepends on a range of values of t (viz., from −T to T), C(τ) may besensitive to changes in polarity of D(t). As a result, cross-correlationnoise such as 1300 may be observed in C(τ). Noise 1300 may make itdifficult to separate line of sight signal components from multipathbecause the noise and line of sight components may be of similarmagnitude.

FIG. 14 shows two illustrative examples of C(τ). Form 1400 is identicalto that shown in FIG. 12. Form 1450 has enhanced peak magnitude relativeto form 1400. Peak magnitude may be enhanced by using a reference signalequal to a concatenation of information symbols that may be present inD(t). For example, a correlation function C′(τ) defined asC′(τ)=∫_(−T) ^(T) D(t)R′(t−τ)dt  (3)may include signal R′(t). R′(τ) may include concatenated D(t)information symbols. For example, R′(t) may be the concatenation of the3 consecutive symbols (each denoted by a “+”) shown in FIG. 12.

FIG. 15 shows a general flowchart of illustrative steps that may beinvolved in some embodiments of the present invention. The steps shownin FIG. 15 are only illustrative and may be performed in any suitableorder. In practice, there may be additional steps or some of the stepsmay be deleted. Some of the steps shown in FIG. 15 may involve receivingwireless signals. Some of the steps shown in FIG. 15 may involveprocessing signals. These and other steps may be performed using anysuitable apparatus, including receivers such as receiver 110 shown inFIG. 1 and apparatus such as those included in TOA estimation device 200shown in FIG. 2.

For clarity, the following discussion will describe the steps shown inFIG. 15 as being performed by “the system,” which is intended to includeany system suitable for performing the steps. The system may receive adata signal at step 1510. The data signal may be received from awireless asset such as wireless asset 130 shown in FIG. 1 or wirelessasset 350 shown in FIG. 3. The data signal may be demodulated at step1520 to yield a demodulated signal. The system may buffer the datasignal at step 1530. Buffered data may be used when informationsequences are detected as described below. The demodulated signal may bedecoded, by correlation and digitization, for example, into a sequenceof decoded binary data at step 1540. The decoded binary data may bebuffered at step 1550.

In some embodiments, the buffered binary data may be analyzed to detectthe presence of a favorable pattern of information symbols in step 1560.In step 1570, a correlation function such as C(τ), including a referencesignal such as R(t), may be evaluated. (Although steps utilizing C(τ)andR(t) are shown and discussed in connection with FIGS. 15–17 for the sakeof simplicity, the scope of FIGS. 15–17 and their description hereinincludes corresponding steps utilizing C′(τ) and R′(t), whenconcatenated information symbols are selected as a reference function,instead of C(τ) and R′(t), respectively.)

At step 1575, correlation signal quality checks may be performed (onC(τ), for example) Correlation signal quality may be quantified using anobjective measure such as signal-to-noise ratio, peak magnitude, or anyother suitable index.

If a correlation signal C(τ) is of sufficient quality, line of sightsignal components may be separated from multipath in step 1580. Step1580 may include step 1585 for leading edge detection. Step 1580 mayinclude step 1590 for channel estimation. Channel estimation may includesuper-resolution techniques such as MUSIC or any other suitable channelestimation technique.

At step 1580, the system may estimate TOA. In some embodiments, thesystem may maximize C(τ) to determine {circumflex over (τ)}. The systemmay define a TOA as {circumflex over (τ)}. In some embodiments in whichit is possible for C(τ) to have negative values, the system may minimizeC(τ) to determine {circumflex over (τ)}. In some embodiments the systemmay define the TOA to be the leading edge of a correlation peak. Forexample, in step 1590, line of sight peak 1502 shown in FIG. 15A may beseparated from multipath peak 1504 (also shown in FIG. 15A). Leadingedge 1503 of line of sight peak 1502 may be defined as the TOA of thecommunication sequence. Peak 1502 may be distinguished from peak 1504because line of sight pulses are received before multipath pulses.(Multipath signals may have longer propagation paths than line of sighsignals.) In some cases, line of sight peak 1502 may overlap or mergewith multipath peak 1504. In these cases, the leading edge of the mergedpulse may be defined as the TOA in step 1585.

FIG. 16 shows illustrative steps that may be involved in a step such as1560 of FIG. 15. The steps are only illustrative and may be performed inany suitable order. In practice, there may be additional steps or someof the steps may be deleted. In step 1610, the system may ascertain ordetermine that a received data signal is TOA-encoded. For example, awireless asset may transmit a data signal that includes a preselectedcommunication sequence that is favorable for TOA estimation. Thepreselected communication sequence may be positioned within a datapacket at a predetermined location (for example, starting at the nth bitof a data packet). In some embodiments, the system may be configured todetect indicators within a data packet that a TOA estimationcommunication sequence is located at a given position within the packet.Preselected communication sequences may include PBCC, CCK, and OFDMsymbols. Preselected information sequences may include PN codes.

When preselected communication sequences are received by the system,step 1610 may be followed by step 1620, in which the reference signalmay be set equal to a sequence of one or more CCK symbols, PBCC symbols,OFDM symbols, or PN codes.

When the system receives a data packet that is not encoded fortime-stamping, data signal monitoring may be performed as shown in step1630. Data signal monitoring may include monitoring a decoded version ofthe data signal (e.g., from step 1550) for the presence of informationsymbol patterns favorable for time-stamping. Each bit of decoded datamay correspond to one information symbol that may be present in acommunication sequence such as D(t) in FIGS. 12 and 13. After detectinga favorable pattern in the bit stream, a reference signal correspondingto the pattern may be selected for correlation with the demodulatedsignal for time-stamping.

For example, the buffer may store N bits, each bit corresponding to asymbol in D(t), in the order in which the N bits were received anddecoded. Thus, if a sequence of bits favorable for time-stamping isdetected in the bit stream, the system may target symbols in thebuffered data signal that correspond to the bit stream sequence fortime-stamping. For clarity, the set of symbols targeted fortime-stamping will be referred to herein as “M.” When M includes morethan one information symbol, the system may correlate on a subset “P” ofM. P may be central subset of M. For example, if M has 5 informationsymbols, the system may be configured to select a reference signal (R(t)or R′(t)) that correlates strongly with P, the three central informationsymbols of M (viz., the second, third, and fourth symbol of M).

In some embodiments, the system may not perform step 1610. In some ofthese embodiments, the system may be configured to correlate a referencesignal with a pre-determined information symbol or symbols in acommunication sequence. For example, the system may be configured toapply a correlation function to the first information symbol in acommunication sequence. In another example, the system may be configuredto correlate using the 2nd–4th information symbols in a communicationsequence. Any suitable symbol or symbols in a communication sequence maybe selected for correlation with a reference signal.

FIG. 17 shows examples of N, D(t), and P that may be involved in step1630 of FIG. 16, and corresponding examples of R(t) that may be involvedin step 1640 of FIG. 16. (For the purpose of this illustration, R(t)refers to single and concatenated reference signals such as thoserepresented in equation 3 as R′(t).)

Each of Examples 1–5 in FIG. 17 includes a buffer having a size N of 12bits holding a segment of decoded data signal D(t). Each symbol in D(t)is represented by a “+” or a “−”, to indicate a positively polarizedsymbol or a negatively polarized symbol, respectively (following theconvention used in FIGS. 12 and 13). M is a set of symbols that may bepresent in D(t). P is a subset of bits that may be present in M andwhich may be targeted for correlation with a reference signal R(t). Eachsymbol in R(t) is represented by a “+” to indicate a positivelypolarized symbol. Although FIG. 17 shows R(t) having only positivelypolarized symbols, R(t) may include one or more negatively polarizedsymbols if necessary. In some embodiments, R(t) may be a sequence ofinformation symbols that is identical to the sequence of informationsymbols present in P.

In Example 1, only isolated positively polarized symbols are present.The system may select a single symbol reference signal R(t) forcorrelation with the single symbol of P. In Example 2, M includes 2symbols. The system may select a single symbol reference signal R(t) forcorrelation with single target symbol P in M. The system may target thetrailing symbol in M to reduce noise (such as cross-correlation noise)in the leading edge of the resulting peak in C(τ) to improve TOAestimation accuracy (in the presence of multipath, for example). InExample 3, M includes three symbols and the system may target centralsymbol P for correlation with a single symbol R(t). It may be beneficialto target a central subset of symbols in M for correlation with an R(t)having fewer symbols than are present in M. This may reduce noise inC(τ) (or C′(τ)). Examples 4 and 5 illustrate the targeting of centralsymbols in M. In Example 4, the system targets two symbols (P) that arecentral to four symbols in M. P may be correlated with an R(t) having 2symbols. In Example 5, the system targets three symbols (P) that arecentral to five symbols in M. P may be correlated with an R(t) havingthree symbols.

In some embodiments, a library of reference signals R(t) may be storedin a look-up table. The look-up table may be indexed by a range ofpossible detected sequences. A detected sequence may thus be used toselect a reference signal that may produce an optimal correlationsignal. Some embodiments may provide rules for prioritizing possiblechoices of R(t) for a given detected sequence in a received data signal.The selection of an appropriate R(t) may produce a correlation signalhaving pulses that have little or no cross-correlation noise such asthose shown in FIG. 14.

FIG. 18 shows a general flowchart of illustrative steps involved in someembodiments of the present invention. The steps shown in FIG. 18 areonly illustrative and may be performed in any suitable order. Inpractice, there may be additional steps or some of the steps may bedeleted. Some of the steps shown in FIG. 18 may involve receivingwireless signals and processing signals. These and other steps may beperformed using any suitable apparatus, including receivers such asreceiver 110 shown in FIG. 1 and apparatus such as those included in TOAestimation device 200 shown in FIG. 2.

For clarity, the following discussion will describe the steps shown inFIG. 18 as being performed by “the system,” which is intended to includeany system suitable for performing the steps. The system may receive adata signal at step 1810. The data signal may be received from awireless asset such as wireless asset 130 shown in FIG. 1 or wirelessasset 350 shown in FIG. 3. The data signal may be demodulated at step1812 to yield a demodulated signal. The demodulated signal may besimilar to that described above in connection with FIG. 15. Thedemodulated signal may be split for parallel processing, which mayinclude parallel correlation, and any other suitable processing,filtering, or buffering steps, at step 1814.

The demodulated signal, which may correspond to D(t) in equations (1) or(3), may be fed simultaneously to multiple correlators in steps 1820,1822, 1824, 1826, and 1828. Steps 1822, 1824, and 1826, may allow1-symbol, 2-symbol, and 3-symbol correlations to be performedsimultaneously. The step 1822 correlation may use a reference signalthat corresponds to R(t) in equation (1). The correlations of steps1824–1828 may use reference signals that correspond to R′(t) in equation(3) because steps 1824–1828 may involve concatenated reference signals.Correlations involving sequences of information symbols longer thanthose in steps 1822–1826 may be performed in step 1828 concurrently withsome or all of steps 1822–1826. In any of steps 1822–1828, the systemmay store a sufficient number of symbols to permit correlation using aset M of target symbols or a subset P of target symbols, as definedabove in connection with FIG. 17.

The system may detect sequences in the demodulated data signal usingsteps 1820 and 1830. The system may decode the demodulated data signalat step 1820 using, for example, a 1-symbol correlator. The decodingcorrelator may be similar to or identical to a 1-symbol correlator thatmay be used in step 1822. As demodulated data stream through the decoderin step 1820, the resulting bits may be stored in a buffer for sequencedetection in step 1830. After a sequence is detected, a correlationsignal (e.g., produced in steps 1822–1828) based on a reference signalknown to correlate strongly with the detected sequence may be selectedin step 1840. Steps 1832–1838 are multipath processing steps (“MPP,” inFIG. 18) that may be used to filter multipath signals out of correlationsignals produced in steps 1822–1828, respectively.

In step 1850, the system may define a TOA estimate as an estimator suchas {circumflex over (τ)} by maximizing the selected correlationfunction. In some embodiments, the correlation function may be definedin a manner that requires minimization to evaluate {circumflex over(τ)}.

In some embodiments, the system may define a TOA estimate in steps1832–1838 using leading edge detection, channel estimation, or acombination thereof. (Leading edge detection and channel estimation arediscussed above, particularly in connection with FIGS. 15 and 15A.) Inthese embodiments, step 1840 may involve selecting a TOA estimate fromthe results of steps 1832–1838. In these embodiments, it may not benecessary in step 1850 to determine a TOA estimator such as {circumflexover (τ)}to define a TOA estimate.

FIG. 19 shows illustrative buffers 1900 (for decoded data), 1922 (forinformation symbols), and 1932 (for information symbols) that may beused to perform some of the steps shown in FIG. 18. Demodulated signal1902 may be passed to decoder 1904, correlator 1920, and correlator1930. Decoder 1904 may pass output 1906 to buffer 1900 in sequencedetector 1907 for detection of a pattern such as 1908. In thisillustrative example, pattern 1908 includes five consecutive identicalbits, including initial bit 1909 and final bit 1910. Bit 1911, having avalue different from those in pattern 1908, may signal the end ofpattern 1908. One-symbol correlator 1920 and three-symbol correlator1930, in respective buffers 1922 and 1932, may concurrently generaterespective correlation signals C₁, and C₃. M₁, P₁, and R₁ and M₃, P₃,and R₃ correspond to M, P, and R shown in FIG. 17 for a one-symbolcorrelator and a three-symbol correlator, respectively.

The detection of pattern 1908 may be used to select one of thecorrelator outputs (e.g., C₁, or C₃) for use in TOA determination. Inthe example shown in FIG. 19, C₃ may be selected because C₃ may beexpected to be stronger than C₁. C₃ may be stronger than C₁ because C₃is based on correlation of a three symbol reference signal (R(t)) with athree symbol subset (P) centered on a five symbol subset (M). Incontrast, C₁ is based on correlation of a one symbol reference signal(R₁(t)) with a one symbol subset (P) centered on a one symbol subset(M).

In some embodiments, sequence detector 1907 may be configured to changethe criteria used to search for an information sequence in decoderoutput 1906. For example, detector 1907 may be programmed to search fora sequence of five consecutive identical symbols. If such a sequence isnot detected after a predetermined number of bits is analyzed (or aftera predetermined period of time has passed, or both) the decoder mayautomatically shift to a search for a pattern that is more likely to befound (e.g., a shorter pattern). In some embodiments, numerous searchstrategies may be used. In some embodiments, detector 1907 may haveprocessing features and buffer capacity suitable for rescanning some orall of decoder output 1906 to identify different bit patterns.

FIG. 20 shows illustrative TOA estimation device 2000 that may be usedto perform some of the steps shown in FIGS. 16 and 18. Radio receiver2010 may be similar to receiver 110 shown in FIG. 1. Radio receiver 2010may be similar to the combination of central processing unit 220 andradio module 230 shown in FIG. 2. Other components of system 2000 mayintegrated into processor 120 of system 100, timing module 240 of system200, or any other suitable signal processing apparatus. System 2000 maybe in communication with a communication network such as network 310shown in FIG. 3.

Antenna 2012 may receive a communication sequence modulated on a carriersignal from a wireless asset such as wireless asset 130 shown in FIG. 1.Radio receiver 2010 may provide baseband in-phase (I) and quadrature (Q)signals, which may be digitized by A/D converters 2014 and 2016,respectively, to carrier tracking circuit 2020. Circuit 2020 may includecarrier signal tracking and/or timing loops to synchronize system 2000timing with the carrier signal. Tracking circuit 2020 may provide thesynchronized communication sequence to chipping code correlator fordecoding the communication sequence. Feedback loop 2032 may providefeedback to circuit 2020 for tracking control. Correlator 2030 mayprovide a correlation signal to decoder 2050. Decoder 2050 may decodehigh data rate modulation symbols received from tracking circuit 2020via path 2035. High data rate modulation symbols may include 802.11 datastructures such as PBCC, CCK, and any other suitable high data ratemodulation symbol. Descrambler 2060 may be present to descramble decodeddemodulated communication sequences. MAC interface 2070 may providedescrambled communication sequences to network resources.

Circuit 2020 may provide the synchronized communication sequence totime-stamping circuit 2040 for TOA estimation. By-pass 2022 may bepresent to permit circuit 2040 to perform tasks substantially inparallel with correlator 2030. Circuit 2040 may include one or more TOAestimate correlators. In some embodiments, circuit 2040 may perform datasignal decoding step 1820 shown in FIG. 18. In some embodiments, circuit2040 may perform steps shown in FIG. 18 such as sequence detection step1830, multi-bit correlation steps 1822–1828, multipath processing steps1832–1836, estimator selection step 1840, TOA data output step 1850, orany other steps suitable for time-stamping. MAC interface 2070 mayreceive a TOA estimate from circuit 2040 and provide the TOA estimate tonetwork resources.

FIG. 21 shows illustrative network 2100 that may be used for identifyingthe location of a wireless asset. Network 2100 may include multiple TOAestimation devices. For the sake of simplicity, only three TOAestimation device A (2110), TOA estimation device B (2120), and TOAestimation device C (2130), are illustrated in FIG. 21. The TOA devicesmay be in electronic communication with cable 2102. A wireless asset maybe located in as yet unidentified location P. The wireless asset maybroadcast a communication sequence that may be received by TOAestimation devices A, B, and C. A TDOA for a given pair of TOAestimation devices can be used to generate a solution set (as discussedabove) that may include point P. For example, TDOA_(BC), which may becalculated for the communication sequence arriving at TOA estimationdevices B and C, may be used to generate a hyperbola defined by thedifference in distances between the wireless asset and each of systems Band C. The relationship between TDOA_(BC) and the difference indistances may be given by

$\begin{matrix}{{{TDOA}_{BC} = {\frac{1}{c}\left( {r_{B} - r_{C}} \right)}},} & (4)\end{matrix}$in which c is the speed of propagation of the communication sequence,r_(B) is the distance between the wireless asset and TOA estimationdevice B, and r_(C) is the distance between the wireless asset and TOAestimation device C. The quantity (r_(B)−r_(C)) may then be used todefine a curve (e.g., a hyperbola) that includes P or an estimatethereof. A second solution set that may include P may be generated, forexample, using a TDOA generated using system pair A and B or system pairA and C. TOA estimates from additional TOA estimation devices (notshown) may be used to generate one or more solution sets for identifyinglocation P.

Master clock 2104 may provide a synchronization signal via cable 2102 tothe TOA estimation devices. The synchronization signal may be used tosynchronize clocks, timers, or counters that may be present in thedevices. The synchronization signal may send a reset pulse to the TOAestimation devices to force device clocks to reset simultaneously. Whencable 2102 includes a power transmission line (e.g., an Ethernet DCpower line), the synchronization signal may be transmitted using thepower line. For example, the synchronization signal may be added orcapacitively coupled to the DC power signal. When cable 2102 includes adata transmission line, the synchronization symbol may be transmittedusing the data transmission line. For example, cable 2102 may include atwisted pair of wires (for example, an Ethernet data transmission line).The synchronization signal may be superimposed on data signals carriedby the twisted pair. At the TOA estimation device the timing and datasignals may be separated using filtering, common mode rejection, or acombination thereof. Different TOA estimation devices may have differentfixed delays. For example, if the lengths of cable between clock 2104and the TOA estimation devices are different, or if processing rates inthe devices differ, TOA's generated by the devices may include offsetseven after synchronization. Offsets may be quantified and TOA estimatesautomatically compensated before a TDOA is calculated. In someembodiments, a beacon may be provided for broadcasting a wirelesssynchronization signal to devices 2110–2130.

FIGS. 22 and 23 show general flowcharts of illustrative steps involvedin some embodiments of the invention related to wireless asset locationidentification using TDOA's. The steps shown in FIGS. 22 and 23 are onlyillustrative and may be performed in any suitable order. In practice,there may be additional steps or some of the steps may be deleted. Someof the steps shown in FIGS. 22 and 23 may involve receiving wirelesssignals and processing signals. These and other steps may be performedusing any suitable apparatus, including system 100 (including, e.g.,receiver 110) shown in FIG. 1 and system 200 shown in FIG. 2.

In step 2210, clocks of network TOA estimation devices to be used forwireless asset location identification may be synchronized to a selectednetwork time signal or counter. In step 2220, a first TOA estimationdevice may receive a communication sequence from the wireless asset andgenerate TOA₁, (a first TOA estimate). In step 2230, TOA₁ may bereferenced to network time. In step 2240, a second TOA estimation devicemay receive the communication sequence and generate TOA₂, (a second TOAestimate). TOA₂ may be referenced to network time in step 2250. In step2260, a TDOA may be calculated using TOA₁ and TOA₂. A set of possiblewireless asset locations may then be generated in step 2270. Steps2210–2260 may be repeated to generate one or more additional solutionsets of possible wireless asset locations.

FIG. 23 shows illustrative steps that may be performed when multiple TOAestimation devices may provide multiple TDOA estimates. In step 2310,TOA estimates generated in connection with a broadcast communicationsequence may collected. In step 2320, a TOA estimation device may bedesignated as a reference TOA estimation device. In step 2330, TOA'sfrom nonreference TOA estimation devices may be used in conjunction withthe TOA generated by the reference system to calculate a TDOA for eachof the nonreference systems. In some embodiments, more than onereference system may designated. In step 2340, each TDOA may be used togenerate an asset location solution set. In step 2350, solution setsthat are physically unreasonable or impossible (“out-of-bounds”),degraded by noise, or otherwise inferior may discarded. In step 2360, ifat least two solution sets remain after selections are made in step2350, wireless asset location may be identified using the remainingsolution sets and a solution estimation method such as least squaresestimation, maximum likelihood estimation, noise-weighted maximumlikelihood estimation, or any other suitable estimation method. If fewerthan 2 solution sets remain after step 2350, the process may proceedwith step 2370, in which no location is identified.

FIG. 24 shows illustrative correlation signals C_(A)(τ) and C_(B)(τ)associated with a communication sequence broadcast by a wireless asset.The communication signal may be received by multiple TOA estimationdevices, each of which may generate a correlation signal. For example,C_(A)(τ) and C_(B)(τ) may be generated using TOA estimation devices suchas A and B, respectively, shown in FIG. 21. C_(A)(τ) and C_(B)(τ) may besimilar to C(τ) shown in FIG. 12. In FIG. 25, C_(A) and C_(B) depend onthe time variables τ_(A) and τ_(B), respectively, which may bereferenced to internal clocks or counters in systems A and B,respectively. These internal clocks may be synchronized so that τ_(A)and τ_(B) are synchronized and referenced to the same standard. C_(A)may have peaks C_(A) ₁ , C_(A) ₂ , C_(A) ₃ , and C_(A) ₄ , for example,corresponding respectively to TOA estimators {circumflex over (τ)}_(A) ₁, {circumflex over (τ)}_(A) ₂ , {circumflex over (τ)}_(A) ₃ , and{circumflex over (τ)}_(A) ₄ . C_(B) may have peaks C_(B) ₁ , C_(B) ₂ ,C_(B) ₃ and C_(B) ₄ , for example, respectively corresponding to TOAestimators {circumflex over (τ)}_(B) ₁ , {circumflex over (τ)}_(B) ₂ ,{circumflex over (τ)}_(B) ₃ , and {circumflex over (τ)}_(B) ₄ . Theestimators may be determined using equation 1. Multiple TDOA's may begenerated using pairs of TOA estimators including estimators from bothsystems A and B. For example, TDOA₁, shown in FIG. 24, may be generatedfrom {circumflex over (τ)}_(A) ₁ and {circumflex over (τ)}_(A) ₂ . TDOA₂may be generated from {circumflex over (τ)}_(A) ₂ and {circumflex over(τ)}_(B) ₁ . Although four TDOA's are shown in FIG. 24, some embodimentsmay generate more than four TDOA's. Although the TDOA's shown in FIG. 24are based on TOA estimates such as {circumflex over (τ)}_(A) ₁ and{circumflex over (τ)}_(B) ₁ , some embodiments may use TOA estimatesbased on leading edge detection, channel estimation, or a combinationthereof for calculating an average TDOA. The TDOA's shown in FIG. 24 maybe referred to as “preliminary TDOA's”. In some embodiments, the averageof the preliminary TDOA's may be used to generate a solution set ofpossible asset locations for the wireless asset as discussed inreference to FIGS. 21–23.

Thus it is seen that apparatus and methods for identifying wirelessasset location in a wireless communication network have been provided.One skilled in the art will appreciate that the present invention can bepracticed by other than the described embodiments, which are presentedfor purposes of illustration and not of limitation, and the presentinvention is limited only by the claims which follow.

1. A mobile unit for providing location identification signals, saidlocation identification signals useful for determining location of amobile asset in a communication network, said mobile unit comprising: areceiver; a transmitter; and a controller; wherein said controller isresponsive to wake up signals received by said receiver to operate saidreceiver to determine if radio frequency energy in a first channel in afirst cell of a cellular network is substantially less than apredetermined threshold, and to cause said transmitter to transmit saidlocation identification signals on said first channel if radio frequencyenergy in said first channel is substantially less than a predeterminedthreshold: wherein said controller is responsive to said receiver, ifsaid radio frequency energy in said first channel is not substantiallyless than said threshold, to cause said receiver to detect the presenceof radio frequency energy in a second channel in a second cell of saidcellular network; and wherein said controller is responsive to saidreceiver, if radio frequency energy in said second channel issubstantially less than a predetermined threshold, to cause saidtransmitter to transmit said location identification information on saidsecond channel.
 2. A mobile unit for providing location identificationsignals, said location identification signals useful for determininglocation of a mobile asset in a communication network, said mobile unitcomprising: a transmitter; a controller for delaying a predeterminedperiod of time; a receiver for detecting the presence of radio frequencyenergy on a first channel in a first cell of a cellular network inresponse to said controller; wherein said controller is responsive tosaid receiver, if said radio frequency energy is substantially less thana predetermined threshold, to cause said transmitter to transmit saidlocation identification signals on said first channel: wherein saidcontroller is responsive to said receiver, if said radio frequencyenergy on said first channel is not substantially less than saidthreshold, to cause said receiver to detect the presence of radiofrequency energy on a second channel in a second cell of said cellularnetwork; and wherein said controller is responsive to said receiver, ifradio frequency energy on said second channel is substantially less thana predetermined threshold, to cause said transmitter to transmit saidlocation identification signals on said second channel.
 3. The mobileunit of claim 2 wherein said transmitter is configured to transmit atleast one information sequence selected for time-of-arrival estimation.4. The mobile unit of claim 2 wherein said transmitter is configured totransmit asset identification information.
 5. The mobile unit of claim 2wherein said transmitter is configured to transmit an 802.11 datapacket.
 6. The mobile unit of claim 2 wherein said receiver comprises anenergy detector.
 7. A method for providing location identificationsignals, said location identification signals useful for determininglocation of a mobile asset in a communication network, said methodcomprising: receiving a wake-up signal from a transmitter in saidnetwork; detecting the presence of radio frequency energy on a firstchannel in a first cell of a cellular network in response to saidwake-up signal; if said radio frequency energy is substantially lessthan a predetermined threshold, transmitting said locationidentification signals on said first channel; if said radio frequencyenergy on said first channel is not substantially less than saidthreshold, detecting the presence of radio frequency energy on a secondchannel in a second cell of said cellular network; and if radiofrequency energy on said second channel is substantially less than apredetermined threshold, transmitting said location identificationsignals on said second channel.
 8. The method of claim 7 wherein saidtransmitting comprises transmitting at least one information sequenceselected for time-of-arrival estimation.
 9. The method of claim 7wherein said transmitting comprises transmitting asset identificationinformation.
 10. The method of claim 7 wherein said transmittingcomprises transmitting an 802.11 data packet.
 11. A method for providinglocation identification signals, said location identification signalsuseful for determining location of a mobile asset in a communicationnetwork, said method comprising: waiting a predetermined period of time;detecting the presence of radio frequency energy on a first channel in afirst cell of a cellular network; if said radio frequency energy issubstantially less than a predetermined threshold, transmitting saidlocation identification signals on said first channel; if said radiofrequency energy on said first channel is not substantially less thansaid threshold, detecting the presence of radio frequency energy on asecond channel in a second cell of said cellular network; and if radiofrequency energy on said second channel is substantially less than apredetermined threshold, transmitting said location identificationsignals on said second channel.
 12. The method of claim 11 wherein saidtransmitting comprises transmitting at least one information sequenceselected for time-of-arrival estimation.
 13. The method of claim 11wherein said transmitting comprises transmitting asset identificationinformation.
 14. The method of claim 11 wherein said transmittingcomprises transmitting an 802.11 data packet.
 15. The method of claim 11wherein said detecting comprises using an energy detector.